The present invention relates to an apex-angle variable prism which is most suitably applied to an apparatus for correcting the blurring of picture from an unsteady hold on a video camera, for example, and a video camera using the same.
In general, when a video camera is shaken due to an unsteady hold on the video camera or the like during a video pickup operation of the video camera, blurring of picture being picked up occurs. Therefore, there has been recently publicly known a picture-blurring correcting apparatus for a video camera in which an apex-angle variable prism is secured to the tip of the lens barrel of a video camera, blurring of picture being picked up or the like is detected by a built-in sensor, and the apex angle of the apex-angle variable prism is varied in accordance with the output of the sensor to incline the optical axis by the angle at which the video camera is shaken, thereby suppressing the blurring of the picture being picked up.
For example, according to an apex-angle variable prism described in Japanese Laid-open Patent Application No. Sho-61-269572, special liquid is sealed between two flat glass members which are covered by bellows, and the apex angle of the apex-angle variable prism is varied by adjusting the apex angle of one flat glass member to bend the optical axis in such a direction as to cancel out the shake amount of the video camera. Further, Japanese Laid-open Patent Application No. Hei-6-70220 or No. Hei-6-281889 discloses an apex-angle variable prism in which a planoconcave lens and a planoconvex lens, which have the same refractivity and spherical surfaces of the same radius of curvature, are confronted to each other so as to keep a slight gap between the spherical surfaces thereof, and the apex angle between the two flat surfaces of the two lens is varied by rotating one lens along the spherical surface thereof. Further, Japanese Laid-open Patent Application No. Hei-6-281889 describes a biaxial rotational driving mechanism in which a planoconcave lens is fixed and a planoconvex lens is rotated around perpendicularly intersecting two axes by a biaxial actuator to rotate the planoconvex lens in the two axial directions along the spherical surface of the planoconcave lens.
However, in the apex-angle variable prism as described in the Japanese Laid-open Patent Application No. Sho-62-26957, since the special liquid is sealed between the two flat glass members covered with the bellows, the viscous resistance of the liquid acts as drag when the angle of one flat glass member is varied. Therefore, the variation speed of the angle of the flat glass member has a limitation, and thus there is a problem that it is difficult to follow a high-speed shaking motion. Further, in the apex-angle variable prism as disclosed in the Japanese Laid-open Patent Application No. Hei-6-70220 or the Japanese Laid-open Patent Application No. Hei-6-281889, since the radius of curvature of the spherical surfaces of the planoconcave lens and the planoconvex lens cannot be set to such a so small value, it is difficult to rotatably dispose these lens around the center of the radius of curvature of the spherical surfaces in a narrow space. Even if the spherical surfaces of these lens are brought into directly sliding contact with each other, the spherical surfaces are damaged, and also the variation speed of the apex angle is suppressed by the sliding resistance of the lenses, so that the response to the high-speed shaking motion is reduced.
The biaxial rotational driving mechanism for rotating the planoconvex lens in the two axial directions along the spherical surface of the planoconcave lens as described in Japanese Laid-open Patent Application No. Hei-6-281889, a horizontal rotational member must use a gyroscope structure for supporting a horizontal rotational member through a vertical first support shaft on a lens support member so as to be rotatable in the horizontal direction and also supporting a vertical rotational member holding the outer periphery of the planoconvex lens on a horizontal rotational member through two horizontal second support shafts so as to be rotatable in the vertical direction. Therefore, the overall biaxial rotational driving mechanism is designed in a large-size and heavy weight structure. Accordingly, the inertia when the planoconvex lens is rotated in the two axial directions is large, and an actuator of large capacity must be used, so that there is a problem that the cost increases remarkably and the response to the high-speed shaking motion is reduced.